IRU3038CS datasheet(4/18 Pages) IRF

4 / 18 page

Rev. 2.0

09/12/02

IRU3038

www.irf.com

BLOCK DIAGRAM

Figure 2 - Simplified block diagram of the IRU3038.

THEORY OF OPERATION

Introduction

The IRU3038 is a fixed frequency, voltage mode syn-

chronous controller and consists of a precision refer-

ence voltage, an uncommitted error amplifier, an internal

oscillator, a PWM comparator, 0.5A peak gate driver,

soft-start and shutdown circuits (see Block Diagram).

The output voltage of the synchronous converter is set

and controlled by the output of the error amplifier; this is

the amplified error signal from the sensed output voltage

and the voltage on non-inverting input of error amplifier(VP).

This voltage is compared to a fixed frequency linear

sawtooth ramp and generates fixed frequency pulses of

variable duty-cycle, which drives the two N-channel ex-

ternal MOSFETs.

The timing of the IC is provided through an internal oscil-

lator circuit which uses on-chip capacitor. The oscilla-

tion frequency is programmable between 200 to 400KHz

by using an external resistor. Figure 12 shows switch-

ing frequency vs. external resistor (Rt).

Soft-Start

The IRU3038 has a programmable soft-start to control

the output voltage rise and limit the current surge at the

start-up. To ensure correct start-up, the soft-start se-

quence initiates when the Vc and Vcc rise above their

threshold (3.3V and 4.2V respectively) and generates

the Power On Reset (POR) signal. Soft-start function

operates by sourcing an internal current to charge an

external capacitor to about 3V. Initially, the soft-start func-

tion clamps the E/A’s output of the PWM converter. As

the charging voltage of the external capacitor ramps up,

the PWM signals increase from zero to the point the

feedback loop takes control.

Short-Circuit Protection

The outputs are protected against the short-circuit. The

IRU3038 protects the circuit for shorted output by sens-

ing the output voltage (through the external resistor di-

vider). The IRU3038 shuts down the PWM signals, when

the output voltage drops below 0.6V.

The IRU3038 also protects the output from over-voltaging

when the control FET is shorted. This is done by turning

on the sync FET with the maximum duty cycle.

Under-Voltage Lockout

The under-voltage lockout circuit assures that the

MOSFET driver outputs remain in the off state whenever

the supply voltage drops below set parameters. Lockout

occurs if Vc and Vcc fall below 3.3V and 4.2V respec-

tively. Normal operation resumes once Vc and Vcc rise

above the set values.

20uA

64uA Max

POR

Oscillator

Error Amp

Error Comp

Reset Dom

POR

0.6V

FbLo Comp

HDrv

Vcc

LDrv

PGnd

Vcc

3.5V

0.2V

0.25V

Bias

Generator

1.25V

POR

Comp

25K

1.25V

V REF

Gnd

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